Method for manufacturing a thin film semiconductor device, method for manufacturing a display device, method for manufacturing thin film transistors, and method for forming a semiconductor thin film.
This invention relates to a method for manufacturing thin film semiconductor devices involving thin film transistors formed in an active matrix type display device in an integrated form. More particularly, the invention relates to a method for forming polycrystalline semiconductor thin film transistors.
Crystallization annealing by laser light has been developed as a part of technologies to proceed with the manufacturing process of thin film semiconductor devices under low temperatures. This technique first irradiates laser light onto a semiconductor thin film of a non-single-crystalline material such as amorphous silicon or polycrystalline silicon with a relatively small grain size, which is formed on an insulating substrate, to locally heat it, and then makes the semiconductor film change into a polycrystal with a relatively large grain size during its cooling process (crystallization). The crystallized semiconductor thin film is used as an active layer (channel region) to integrally build thin film transistors. By employing this crystallization annealing, thin film semiconductor devices can be made under low process temperatures, and this enables the use of inexpensive glass substrates instead of expensive quartz substrates with a higher heat resistivity.
In crystallization annealing, line-shaped laser light normally elongated in the scanning direction is intermittently irradiated while making its shots partly overlapped. By overlapping shots of the laser light, the semiconductor thin film can be crystallized more uniformly. Crystallization annealing which uses line-shaped laser light (line beam) is schematically shown in FIG. 1. Laser light 50 shaped into a line extending in the Y direction of an insulating substrate 1 of glass, for example, is irradiated onto the surface of the insulating substrate 1 having formed a semiconductor thin film. In this process, the insulating film substrate 1 is moved in the X direction relative to the irradiated region. In this example, a line beam 50 released from an excimer laser source is irradiated intermittently in a partly overlapped fashion. That is, the insulating substrate 0 is scanned in the X direction relative to the line beam 50 through a stage. Crystallization annealing is conducted by moving the stage after each shot by a pitch smaller than the width of the line beam 50 to ensure that the line beam 50 can irradiate the entire surface of the insulating substrate 1. Excimer laser sources used in conventional crystallization annealing release pulses of 100 Hz or higher frequency, and the pulse width of each line beam is smaller than 50 ns.
Thin film semiconductor devices integrating thin film transistors are used in many active matrix type display devices, or the like. In order to realize display devices excellent in image quality, it is important to integrate thin film transistors having good operation properties all over the substrate. For this purpose, it is necessary to uniformly stack a semiconductor thin film of a polycrystal with a relatively large grain size. Additionally, needless to say, when taking the production yield into consideration, crystal grains having a large grain size must be uniformly built in all over the substrate. There are some methods for obtaining such a polycrystal, such as increasing the laser light irradiation energy, increasing the number of shots of overlapping irradiation, and making a crystal core in an amorphous semiconductor thin film before irradiation of laser light, for example. Even with these methods, however, no techniques have been successful in making sufficiently large, uniform crystal grains. Therefore, no system-on-panels remarked as the final target of low-temperature-process have been realized heretofore. A system-on-panel pertains to a device including built-in peripheral devices such as a video driver and a timing generator on a common substrate in addition to a switching element for driving pixels and thin film transistors used as horizontal scanners and vertical scanners. For realizing a system-on-panel, mobility u of individual thin film transistors has to be increased to 80 cm2/Vxc2x7s through 300 cm2/Vxc2x7s. For this purpose, it is necessary to further decrease the grain size of polycrystalline semiconductor thin film.
As shown in FIG. 1, when the line beam is irradiated to partly overlap between one shot and another, streaks appear to extend in the direction (Y direction) normal to the forward direction (X direction). In a microscopic view, these streaks are unevenness in crystal grain size. When the thin film transistors are integrated on the insulating substrate, unevenness in crystal grain size is observed as unevenness of the operation property, and it is therefore difficult to fabricate a display device ensuring a high quality throughout the entire surface of the insulating substrate. The first object of the invention is directed to solution of these problems of conventional techniques and provides a method for obtaining a polycrystal having a uniform, large grain size all over the surface of the insulating substrate by irradiating laser light onto a semiconductor thin film.
Next explained is another problem the invention intends to overcome. Thin film transistors are widely used as switching elements of active matrix type display devices. Especially as the semiconductor thin film to form the active layer of thin film transistors, polycrystalline silicon has been used for years. Polycrystalline silicon thin film transistors are used not only as switching elements, but they can be used also as circuit elements, and peripheral driving circuits can be built in together with pixel-driving switching elements on a common substrate. However, to form these peripheral driving circuits, high-performance thin film transistors are required. Particularly, their mobility is desired to be high.
Solid phase growth has been known for years as a technique for making high-quality polycrystalline silicon on an insulating substrate. This is a method which makes a silicon film as a precursor film by LP-CVD and then anneals it. Regarding the relation between conditions of deposition by LP-CVD and subsequent heating and the crystal grain size, it is known desirable to form amorphous silicon at a temperature not higher than 580xc2x0 C. and anneal it at about 600xc2x0 C., for example, in order to obtain polycrystalline silicon with a large grain size. In case of solid phase growth by heating, if an amorphous silicon film is annealed at 600xc2x0 C. for 12 hours, for example, the crystal grain size reaches 100 through 2000 nm. In general, the larger the grain size, the higher the mobility. However, in solid phase growth by annealing, crystal patterns are not constant, and a lot of twin crystal defects and dislocation defects are observed in crystal grains through a crystal image. Because of these defects, although polycrystalline. silicon obtained by solid phase growth has a large grain size, its mobility is only around 70 cm2/Vxc2x7s.
Laser annealing has also been used as a technique for making high-quality polycrystalline silicon. With this method, silicon thin films can be crystallized at relatively low temperatures without heating the entire substrate so high. When laser light is irradiated onto a silicon thin film to be crystallized, the energy is absorbed only by the very surface of the silicon thin film. Thereafter, the inner portion of the thin film melts due to heat conduction, and re-crystallizes during its cooling process. In polycrystalline silicon films made in this manner, crystal grains are distributed about uniformly. Also when reviewing its lattice image, crystal defects are less than those by solid phase growth. However, in the case of laser annealing, the crystal grain size is relatively as small as 200 through 300 nm, a lot of crystal grain boundaries exist, and the mobility is around 40 cm2/Vxc2x7s.
There is a recently developed technique for reducing crystal defects by adding laser annealing by excimer laser light after solid phase growth by annealing. It is disclosed, for example, in Japanese Patent Laid-Open Publication No. sho 62-104021 and Japanese Patent Laid-Open Publication No. hei 7-302913. However, even with this method, mobility of thin film transistors has not been increased sufficiently. The second object of the-invention is to obtain a higher mobility by optimizing conditions of laser annealing after solid phase growth. Additionally, in the technology combining solid phase growth and laser annealing, the crystal grain size of the polycrystalline silicon thin film is basically determined by solid phase growth. In order to obtain thin film transistors with a higher mobility, the grain size of the poly crystal must be increased more. This is also the second object of the invention. Furthermore, it is the second object of the invention to provide a silicon thin film having a still higher quality by employing a deposition method as a replacement for solid phase growth in combination with laser annealing.
Here is explained still another problem of conventional techniques. Heretofore, solid phase growth or annealing has been used as a method for making a polycrystalline semiconductor thin film to be used as the active layer of thin film transistors. In solid phase growth, although crystal grains contained in the polycrystalline semiconductor thin film grow to a size as large as about 1 xcexcm, it contains a lot of crystal defects such as dislocation. Therefore, mobility of the thin film transistors is 100 cm2/Vxc2x7s or less in terms of the N channel. In contrast, in the case of crystallization by laser annealing, although crystal grains contain less crystal defects, since the crystal grain size can increase only to 200 through 300 nm, approximately, mobility of the thin film transistors also remain in the level equivalent to or lower than that by solid phase growth. There is a recently proposed method for repairing crystal defects by conducting laser annealing after solid phase growth, and this method can improve the mobility to approximately 130 cm2/Vxc2x7s. However, this method uses laser annealing only for repairing crystal defects, and since the crystal grain size of the polycrystalline semiconductor thin film is determined by solid phase growth in the preceding step, further increase of the crystal grain size cannot be expected. In order to obtain thin film transistors whose mobility exceeds 300 cm2/Vxc2x7s, more increase of the crystal grain size is indispensable, and attainment of this requirement is the third object of the invention.
To accomplish the first object of the invention, a method for manufacturing a thin film semiconductor device according to the invention basically includes a deposition step for making a non-single-crystalline semiconductor thin film on a surface of an insulating substrate, an annealing step for irradiating laser light to once heat and melt the non-single-crystalline semiconductor thin film and then changing it into a poly crystal in its cooling process, and a processing step for integrally forming thin film transistors using the semiconductor thin film of the polycrystal as the active layer. As a particular feature, in the annealing step, laser light having the pulse width of 50 ns or more is irradiated onto the semiconductor thin film by using an excimer laser source. In the annealing step, preferably after the laser light is shaped to have a rectangular cross section whose each side is not shorter than a predetermined length (for example 10 mm), the surface of the semiconductor thin film is irradiated sequentially by moving the laser light step by step so that the sides of the rectangular cross section partly overlap. In this case, frequency of irradiation of the laser light per each stepping movement is selected so that the frequency of overlapping irradiation of the laser light onto the same part of the surface of the semiconductor thin film is a predetermined number.
According to the invention, for making thin transistors using the polycrystalline semiconductor of polycrystalline silicon, for example, as the active layer on the insulating substrate in a low-temperature process, amorphous silicon is first made on the insulating substrate by low-pressure chemical vapor deposition (LP-CVD), plasma CVD or sputtering. After that, by using an excimer laser light source, the laser light is irradiated onto the semiconductor thin film to change the amorphous silicon into polycrystalline silicon. In some cases, polycrystalline silicon having a relatively small grain size may be changed into polycrystalline silicon having a relatively large grain size by irradiation of the laser light. To generalize them altogether, amorphous silicon and polycrystalline silicon having a small grain size are called non-single-crystalline silicon, and in this embodiment, non-single-crystalline silicon is changed into polycrystalline silicon by irradiation of the laser light. Unlike the conventional techniques, in the present invention, laser light released from an excimer laser source having the pulse width of 50 ns or more is shaped into a rectangle by an appropriate optical system, and it is irradiated onto the semiconductor thin film step by step. For example, the laser light is shaped by an optical system to have a rectangular cross section of 10 mmxc3x9710 mm or larger in terms of the irradiated area of the semiconductor thin film. In this embodiment, laser light is irradiated while moving it step by step to partly overlap it so that the same position is irradiated by the laser light at least two times at least in a part of the entire area of the insulating substrate. By employing this mode of irradiation, it is possible to make a polycrystalline semiconductor thin film having a uniform, large grain size sufficiently acceptable as the active layer of high-mobility, high-performance thin film transistors. More specifically, polycrystalline silicon having the grain size exceeding 300 to 1000 nm can be made uniformly throughout the entire surface of the substrate.
To attain the second object of the invention, the following means is employed. That is, the invention relates to a method for manufacturing a semiconductor transistor, which forms on an insulating substrate a multi-layered structure basically including a semiconductor thin film, gate insulating film stacked on one surface of the semiconductor thin film and gate electrode stacked on the semiconductor thin film via the gate insulating film. According to one aspect of the invention, the thin film transistor is manufactured in the following process. First conducted is a forming step to form on the insulating substrate the semiconductor thin film containing polycrystalline grains. More specifically, a deposition step is conducted to stack on the insulating substrate an amorphous semiconductor thin film or a polycrystalline semiconductor thin film made up of crystal grains of a relatively small grain size. Next conducted is a solid phase growth step to grow crystal grains with a relatively small grain size in solid phase by annealing the semiconductor thin film. Alternatively, a semiconductor thin film containing polycrystalline grains may be stacked directly on the insulating substrate by chemical vapor deposition using a catalyst. After that, a laser annealing step is conducted to remove residual defects in the crystal grains by using laser light in the form of pulses having the emission time of 100 ns or more and irradiating the semiconductor thin film with an energy not inviting destruction of crystal grains of large sizes. Preferably, in the laser annealing step, laser light is irradiated onto the semiconductor thin film with the energy of 500 to 600 J/cm2.
According to another aspect of the invention, the thin film transistor is manufactured in the following process. First conducted is a deposition step to stack on the insulating substrate an amorphous semiconductor thin film or a polycrystalline semiconductor thin film made up of crystal grains with a relatively small grain size. There follows the solid phase growth step in which the semiconductor thin film is annealed to grow crystal grains with a larger grain size in solid phase. Next comes the laser annealing step in which by using laser light in the form of pulses, energy as moderate as not causing destruction of large size crystal grains is irradiated onto the semiconductor thin film to remove residual defects in the crystal grains. After that, an additional solid phase growth step is conducted to anneal the semiconductor thin film again and grow crystal grains with a still larger grain size in solid phase. Preferably, after the additional solid phase growth step, moderate energy not inviting destruction of large-size crystal grains is irradiated again onto the semiconductor thin film by using laser light in the form of pulses, and an additional laser annealing step is conducted to remove defects produced in the additional solid phase growth step.
This aspect of the invention involves a method for making a semiconductor thin film. That is, the method for making a semiconductor thin film according to the invention comprises a forming method for forming a semiconductor thin film on an insulating substrate at a temperature not higher than 400xc2x0 C. by chemical vapor deposition using a catalyst, and a laser annealing step for improving the quality of the semiconductor thin film by irradiating laser light in the form of pulses having emission time not shorter than 100 nm on the insulating substrate. More specifically, in the forming step, a polycrystalline semiconductor thin film containing crystal grains is formed by chemical vapor deposition using a catalyst, and in the laser annealing step, laser light with an energy not inviting destruction of the crystal grains is irradiated to remove defects existing in the crystal grains and thereby improve the quality of the semiconductor thin film. Preferably, the forming step forms a semiconductor thin film containing hydrogen by 1% or less and having a thickness not thicker than 50 nm is formed on the insulating substrate by chemical vapor deposition using a catalyst. Preferably, this forming step forms the semiconductor thin film in a reaction chamber which can be evacuated, and the laser annealing step irradiates laser light on the insulating substrate without breaking the evacuated condition of the reaction chamber. If so desired, the forming step and the laser annealing step are repeated alternately until the semiconductor thin film grows to a predetermined thickness.
According to one aspect of the invention, the polycrystalline semiconductor thin film having large-size crystal grains is obtained by solid phase growth using annealing or chemical vapor deposition using a catalyst. After that, laser annealing is conducted to remove residual defects in the crystal grains. By increasing the grain size by solid phase growth, etc. and removal of defects by laser annealing, mobility of the thin film transistors can be increased. In this case, when laser annealing is conducted by using the pulse-mode laser light having emission time (relaxation time) not shorter than 100 ns, crystal defects can be removed efficiently. Therefore, removal of defects can be improved significantly by using laser pulses having a longer emission time than conventional ones. To maintain the crystal grain size obtained in the solid phase growth, it is important that the energy applied to the semiconductor thin film in the laser annealing step be controlled in a level not inviting destruction of the large-size crystal grains (for example, 500 through 600 cm2/Vxc2x7s). In the laser annealing, by removing residual crystal defects without producing new crystal defects, a high mobility can be attained. In the other aspect of the invention, after the laser annealing step, crystal grains with a still larger grain size are grown in solid phase by annealing the semiconductor thin film again. As a result, a further improvement of the mobility can be attained. By conducting laser annealing after the first solid phase growth, crystal defects decrease, and a stress in the thin film is alleviated. By conducting the second solid phase growth in this status, the crystal grain size is increased efficiently. In the other aspect of the invention, the semiconductor thin film suitable as the active layer of thin film transistors is made by combining chemical vapor deposition using a catalyst (catalytic CVD) and laser annealing. Catalyst CVD is capable of forming the polycrystalline semiconductor thin film of polycrystalline silicon, for example, at a low temperature not higher than 400xc2x0 C. By processing this semiconductor thin film by laser annealing, defects contained in the crystal grains can be removed. Since catalytic CVD and laser annealing are low-temperature processes, it is possible to make thin film transistors in a low-temperature process while maintaining the property of the thin film transistors.
Furthermore, to attain the third object of the invention, the following means is employed. That is, there is provided a method for manufacturing a thin film transistor, which forms on an insulating substrate a multi-layered structure including a semiconductor thin film, a gate insulating film stacked on one surface of the semiconductor thin film, and a gate electrode stacked on the semiconductor thin film via the gate insulating film, and the method comprises a forming step for forming on the insulating substrate a semiconductor thin film containing polycrystalline grains, and a laser annealing step for irradiating laser light in the mode of pulses having emission time not shorter than 50 ns and thereby removing residual defects in the crystal grains to increase the size of the crystal grains. Preferably, in the laser annealing step, irradiation of the pulse-mode laser light is repeated a number of times necessary for the crystal grains to grow to a predetermined size. In the laser annealing step, the pulse-mode laser light is repeatedly irradiated in a cycle not shorter than ⅕ Hz. Still in the laser annealing step, the laser light is irradiated onto the semiconductor thin film under the energy density of 400 through 600 cm2/Vxc2x7s. Further, the laser annealing step uses pulse-mode laser light having emission time not shorter than 100 ns. Furthermore, the laser annealing step irradiates laser light having an irradiation area not smaller than 5 cm2 onto the semiconductor thin film. The forming step includes a deposition step for stacking an amorphous semiconductor thin film or a polycrystalline thin film made of crystal grains with a relatively small grain size on an insulating substrate, and a solid phase growth step for annealing the semiconductor thin film to grow crystal grains with a larger grain size in solid phase. Alternatively, the forming step stacks a semiconductor thin film containing polycrystalline grains on the insulating substrate by chemical vapor deposition using a catalyst.
According to the invention, in the manufacturing process of a thin film transistor, the semiconductor thin film containing polycrystalline grains is formed on the insulating substrate by solid phase growth or catalytic chemical vapor deposition. After that, by repeatedly irradiating pulse-mode laser light, residual defects in the crystal grains are remedied, and the crystal grains are enlarged. As a result, a high-mobility semiconductor thin film can be obtained. Especially when the number of times of irradiation of pulse-mode laser light is determined adequately, a polycrystalline semiconductor thin film containing crystal grains with a predetermined size can be made.